Attorney General William Barr declined on Thursday to appear before the House Judiciary Committee for a hearing concerning special counsel Robert Muellers Russia inquiry.

Rep. Jerry Nadler (D-New York), the committee’s chairman, accused Barr of “trying to blackmail” his panel in comments to reporters on Wednesday. Nadler also said he could release a subpoena for Barr to testify should the attorney general continue to refuse to cooperate. A contempt citation could even potentially follow at some point if a subpoena is also ignored.

Congressional Democrats are already angered by what they view as Barr’s allegiance to President Donald Trump in the Russia probe. This sentiment has been further fueled by Mueller’s revelation this week that he was upset about Barr mischaracterizing parts of the special counsel’s full report, which was released last month. Some Democrats have even called for the attorney general to step down.

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The Justice Department objected to the format of Barr’s scheduled hearing, where both Democratic and Republican lawmakers would be set to question the attorney general. Some GOP congressman have been accused by Democrats of siding with Barr and Trump regarding the Mueller report.

“Unfortunately, even after the attorney general volunteered to testify, Chairman Nadler placed conditions on the House Judiciary Committee hearing that are unprecedented and unnecessary,” Kerri Kupec, a Justice Department spokeswoman, said in a statement. “Chairman Nadler’s insistence on having staff question the Attorney General, a Senate-confirmed Cabinet member, is inappropriate.”

Barr notably exonerated Trump of obstruction of justice in his summary, even though Mueller clearly said that although there was insufficient evidence to charge the president with a crime, he was not cleared of obstructing justice either.

The attorney general said Trump had faced “two years of allegations that have now been proven false.”